a) Field of the Invention
The invention is directed to a convergence tube for stereomicroscopes, in which the two observation beam paths are inclined relative to one another at a convergence angle .alpha. and image inversion elements are provided in both observation beam paths.
b) Description of the Related Art
Stereomicroscopes essentially comprise an objective, a tube system with image inversion elements, and oculars. Usually, there is a Galilean telescope system with a fixed or a variable (zoom) magnification arranged between the objective and the tube system. The distance between the two optical axes of the tube objectives is the stereo base.
Two characteristic types of tube have been developed: first, the parallel tube in which both optical axes emerge parallel to each other; second, the convergence tube in which the optical axes are presented to the observer at an inclination relative to each other of a convergence angle .alpha.. The convergence angle .alpha. corresponds to the viewing direction of the eyes accommodated to an object at a distance of about 250 mm, and usually has a value of between 14.degree. and 16.degree..
While the ergonomic design of the convergence tube has proven itself based on its advantages for the observer, it is more complicated in terms of optics and also, therefore, in technical respects relating to manufacture compared to the parallel tubes, since the convergence of the observation beam paths demands that the image inversion elements be specially designed and positioned relative to one another in a special way.
The convergence of the observation beam paths is essentially achieved according to two basic principles. In the first one, the convergence tubes are outfitted with the conventional image-erecting Porro prisms which are preceded by optical wedges, e.g., achromatic wedges.
In the second principle, the convergence tubes are provided with modified Porro prisms. For example, a 90-degree or right-angle prism is arranged in each beam path, followed by at least one prism deviating from 90.degree.. In this connection, as a rule, the 90-degree angle is deviated from by half the convergence angle .alpha..
These two basic principles form the basis for design variants in which some or all of the prisms are replaced by mirrors.
The insertion of optical wedges as well as the manufacturing of prisms or mirror subassemblies with a deflection angle deviating from 90.degree. is uneconomical with regard to manufacturing techniques.